Pan flashing protects against water damage
Pan flashing can be made from plastic sheeting, EPDM membrane, or something more durable, such as lead-coated copper, or even stainless steel. Nearly as important as head and pan flashing is how it’s tied into the wall behind the siding. Leaking water should roll down to the pan, lip over to a drainage plane and out the bottom. Water that leaks into wall cavities causes rot and mold.
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The first thing I want to show you is the flexible material, because this stuff is very easy to use, so it’s more costly than the straight nonflexible flashing. The first thing I do when I do a sill pan is I precut the material so it wraps up the sides of the jamb somewhere between 4 and 6 inches high. Then I want to remove just part of the release sheet.
Work from the inside, out
Many of the manufacturers have pre-embossed strips along the back, so you don’t have to remove all of the release sheet, just part of it. So I’m going to remove the main part of the back and leave the last strip on the front. I find it’s easiest to work with the flashing tape from the inside of the rough opening rather than the outside, but if you find differently, by all means; it can be done either way.